From 6ab66500e69b7a406f863739e23489213ce7129f Mon Sep 17 00:00:00 2001
From: Ben Bridle <bridle.benjamin@gmail.com>
Date: Sun, 24 Dec 2023 17:57:26 +1300
Subject: First commit

---
 src/lib.rs | 83 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)
 create mode 100644 src/lib.rs

(limited to 'src/lib.rs')

diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..2992069
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,83 @@
+#![feature(iter_array_chunks)]
+
+pub fn encode(bytes: &[u8]) -> String {
+    let remainder = bytes.len() % 3;
+    let bytes = match remainder {
+        0 => bytes.iter().chain(&[0u8;0]),
+        1 => bytes.iter().chain(&[0u8;2]),
+        2 => bytes.iter().chain(&[0u8;1]),
+        _ => unreachable!(),
+    };
+    let mut string = String::new();
+    let mut triples = bytes.array_chunks::<3>();
+    let mut n = 0u32;
+    let mut hextets = [0u32;4];
+    while let Some(triple) = triples.next() {
+        for i in 0..3 {
+            n <<= 8;
+            n += *triple[i] as u32;
+        }
+        for i in (0..4).rev() {
+            hextets[i] = n & 0b111111;
+            n >>= 6;
+        }
+        for i in 0..4 {
+            let symbol = hextets[i];
+            let code = match symbol {
+                 0..=25  =>     symbol + 65,  // A..Z
+                26..=51  =>     symbol + 71,  // a..z
+                52..=61  =>     symbol -  4,  // 0..9
+                62       =>              43,  // '+'
+                63       =>              47,  // '/'
+                _        =>  unreachable!(),
+
+            };
+            string.push(char::from_u32(code).unwrap());
+        }
+    }
+    match remainder {
+        0 => {                                                   string},
+        1 => {string.pop(); string.pop(); string.push_str("=="); string},
+        2 => {              string.pop();      string.push('='); string},
+        _ => unreachable!(),
+    }
+}
+
+pub fn decode(string: &str) -> Result<Vec<u8>, String> {
+    if string.len() % 4 != 0 {
+        return Err("Input string length is not divisible by 4".into())
+    }
+
+    let mut bytes: Vec<u8> = Vec::new();
+    let mut n = 0u32;
+    let mut padding = 0;
+
+    for (i, c) in string.chars().enumerate() {
+        let code: u32 = c.into();
+        if padding > 0 && code != 61 {
+            return Err(format!("Character at position {} is a non-terminating padding character", i-1));
+        }
+        let hextet = match code {
+            65..=90  =>        code - 65,  // A..Z
+            97..=122 =>        code - 71,  // a..z
+            48..=57  =>        code +  4,  // 0..9
+            43       =>               62,  // '+'
+            47       =>               63,  // '/'
+            61       => {padding += 1; 0}, // '='
+            _ => return Err(format!("Character '{c}' at position {i} is not a member of the base64 alphabet")),
+        };
+        n <<= 6;
+        n += hextet;
+        if i & 0b11 == 3 {
+            bytes.extend(&n.to_be_bytes()[1..4]);
+            n = 0;
+        }
+    }
+    if padding > 2 {
+        return Err(format!("Input string contains more than 2 terminating padding characters"))
+    }
+    for _ in 0..padding {
+        bytes.pop();
+    }
+    Ok(bytes)
+}
-- 
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